This invention relates to a process of producing strip or wire, which consists of a monotectic aluminum-silicon alloy comprising a matrix consisting of aluminum and an aluminum-silicon eutectic system and 1 to 50% by weight lead or bismust included in said matrix, which strip or wire has been continuously cast at a high casting velocity and a high cooling rate from a molten material which has been heated to a temperature above the segregation temperature, and which strip or wire has been subjected to plastic deformation and to a heat treatment.
When molten monotectic alloys in which the densities of the segregated liquid phases differ greatly and which have a large segregation temperature interval are heated to temperatures above the segregation temperature, gravitation will result at temperatures near the miscibility gap in a sedimentation and coagulation of the minority phase, which has a higher specific gravity and consists of droplets. In accordance with Stoke's law the sedimentation velocity is proportional to the square of the droplet diameter. For this reason, droplets which differ in diameter will promote the frequency at which droplet collisions and droplet amalgamations occur so that the sedimentation will be accelerated further. But a uniform dispersion of spherical particles which are small in diameter in the matrix of monotectic alloys can be achieved in that the molten material is continuously cast in a vertical direction at a relatively high casting velocity and a relatively high cooling rate to form a strip or wire which has a thickness or diameter from 5 to 20 mm so that a very steep temperature gradient is maintained before the solid-to-liquid phase boundary. As a result the difference between the segregation and solidus isotherms within the system and also the sedimentation path length will be as small as possible. The temperature interval and the path length interval are determined by the isotherms of the segregation temperature and by the temperature which is reached during the monotectic reaction and at which the matrix phase solidifies and as it solidifies includes the still liquid second phase in its then existing distribution. That process is particularly suitable for the production of cast strip and cast wire made of monotectic aluminum-silicon alloys which comprise a matrix consisting of aluminum and an aluminum-silicon eutectic system and 1 to 50% by weight lead or bismuth, which are included in said matrix as a minority phase consisting of fine droplets.
But the dimensions and/or the mechanical technological properties of such a cast structure often do not comply with the requirements set forth and for this reason the cast strip or the cast wire is subjected to a rolling treatment and/or a heat treatment in order to optimize the properties of the material. By the rolling of such a cast structure, the originally spherical lead or bismuth phase is deformed to constitute elongate platelets. But such elongate inclusions will adversely affect the mechanical load-carrying capacity and the technological properties of the material and for this reason a material having the desired properties cannot be produced unless the elongate platelets are transferred to compact structures; this may be effected by a succeeding heat treatment.
A conventional process for transforming and subsequently spheroidizing a disperse low-melting minority phase comprises the prolonged heating of the monotectic alloy to a temperature above the melting temperature of the low-melting minority phase. In that case the minority phase will be transformed and spheroidized by dissolving and transfer processes involving the matrix metal preferably within the molten phase because the solubilities and diffusion coefficients are much higher in molten materials than in solids.
The requirements set forth are not met by monotectic aluminum-silicon alloys, in which the low-melting liquid phases lead and bismuth are included in a matrix consisting of aluminum and an aluminum-silicon eutectic system because the solubilities of molten lead and molten bismuth in aluminum and also the diffusion coefficients of aluminum and silicon in lead and bismuth are very low so that a comparatively very long heat treatment will be required for a transformation and spheroidization of the minority phase consisting of lead and bismuth. The lead phase and the bismuth phase melt at temperatures of 330.degree. and 270.degree. C., respectively. Thereafter the aluminum-silicon eutectic system melts in a monotectic four-phase reaction at 570.degree. and 580.degree. C., respectively, and the aluminum matrix is finally melted.
It has been disclosed in the periodical Metall 36, No. 9/1982, pages 970 to 976, that in a monotectic aluminum-lead alloy a fine and uniform distribution of the lead phase, which is not soluble in solid aluminum and which consists of elongate filaments in the cast strip which has been rolled, can be achieved if tin is included in the aluminum-lead alloy. That measure will increase the solubility and will accelerate the diffusion of lead in aluminum. Because the presence of tin will strongly decrease the melting temperature of the lead, that measure of alloy technology cannot be adopted if the aluminum-lead-tin material will be subjected to thermal loads in use.